Get a Grip on Automation

Shops must match robot gripper size and style to the application at hand.

Robots and some automation systems can be useless if they do not have the correct part-gripping device.

Like a human hand, grippers hold, tighten, handle and release objects/parts. And, like hands, grippers come in different styles and sizes.

Shops have to familiarize themselves with basic gripper types and what they can do so that the shop can select the proper model for its application.

Compressed air, supplied to the cylinder of a gripper body, forces a piston up and down to actuate a mechanical linkage that opens and closes the gripper jaws. There are three primary gripper-jaw motions – parallel, angular and toggle – which refer to the gripper’s jaw motion in relation to its body.

Parallel gripper jaws move in a parallel the gripper body and can be applied to applications that, typically, require more accuracy than other style grippers.

An angular gripper’s jaws open and close around a central pivot point, moving in a sweeping or arcing motion. They often are used when space is limited or when the jaws need to move up and out of the way.

The pivot point jaw movement of a toggle gripper acts as an overcenter toggle lock to provide a high grip force-to-weight ratio. This mechanism will remain locked even if air pressure is lost.

The most popular style of angular, parallel and toggle grippers have two jaws to provide two mounting locations for the fingers that contact the part to be grasped. The jaws move in a synchronous motion, opening and closing toward the central axis of the gripper body.

Three-jaw parallel and toggle grippers are more specialized, and provide three finger-mounting locations. The jaws move the same way as those of a two-jaw gripper, but three jaws provide more contact with parts and more accurate centering than twojaw models.

Geometries of parts to be grasped, processes to be performed, part orientations and the physical space that is available determine whether external or internal gripper options are used. External gripping is the most common way to hold parts, typically using the closing force of the gripper to hold the part.

Internal gripping is used when part geometries allow or require it, and when the processes that are to be performed require access to the outside part surfaces. With this internal gripping, the gripper’s opening force typically secures parts.

Once a shop determines the appropriate gripper for its application, it must consider the best tooling and finger design for it. Fingers actually make contact with the parts that are handled, and the size and grip force needed for the application can be greatly reduced if they are designed correctly.

The encompassing or retention finger shape is preferred for gripping because that shape increases stability and reduces the necessary grip force. However, the encompassing or retention finger shape requires additional jaw travel, and that travel must be taken into consideration for the application.

Like a human wrist, grippers often need to rotate the objects they are holding. They do so with rotary actuators. Rotary actuators are available in a wide variety of models that have different sizes, torques, and rotation angles.

Rotary actuators convert air pressure from a linear motion to a rotating motion via a rack and pinion mechanism.

Supplied air pressure pushes the piston that is attached to a straight set of gear teeth called a rack, in a linear motion. As the piston moves, it pushes the rack in a linear motion.

The gear teeth of the rack mesh with the circular gear teeth of a pinion, forcing the pinion to rotate. The pinion can be rotated back into the original position by supplying air pressure to the opposite side of the air cylinder.

The pinion connects to a shaft that protrudes from the rotary actuator’s body, and the shaft can connect to various tools or grippers.